Backpack necessary for 3 weeks in Thailand?

I"m traveling to Thailand, a few days in Bangkok, a day in Sukhotai, 4-5 in Chiang Mai/Northern area, and then a week in the south (ko samui area). I plan on staying at places with sheets/private bathrooms/a/c. Do I need to get a backpack like everyone else, or is my big travel bag with a shoulder strap good enough for the trek to/fro from bus/train/air stations and the lodging area.

You are going to very main stream destinations popular with the backpacker brigade and the suitcase brigade so your travel bag should be fine.

Agree with kalki. Or just a small backpak, say 30litr, since it's more comfortable than a bag...